1. Field of the Invention
This invention pertains in general to a system and method for the accurate estimation of the position of and underwater vehicle and more particularly to a system and method for the estimation and tracking of an underwater vehicle""s position using a combination of bathymetry data, and the underwater vehicles dynamics data in relation to a position marker.
2. Description of the Prior Art
Under water vehicles are in common use for surveying and locating objects in the ocean due to their relatively low operation cost. To be useful however, the position of an underwater vehicle must be known so that the data collected by the towed vehicle""s sensors can be georectified. Due to ocean dynamics, currents, waves, temperature fluctuation etc., acting on underwater vehicle""s tether or on the vehicle itself, a submerged vehicles position relative to the towing vehicle or some other georectified point is often difficult to determine. Since the ocean is opaque to high-frequency electromagnetic signals, the global positioning system (GPS) generally cannot be used for submerged vehicle positioning.
Current methods for determining the position of a underwater vehicle include the lay-back model (See, FIG. 1), the long base line (LBL) model (See FIG. 2), the short base line (SBL) model (See FIG. 3), the inverted SBL system and the Localization System (LOST)(See FIG. 4). These system are generally configured to manage the position of a tethered or towed vehicle and are ill equipped for an remote or untethered underwater vehicle.
With the lay-back model method the underwater vehicle is tethered and is assumed to be directly behind the towing vehicle and the lay-back (distance from the towing vehicle) is assumed to be a fixed multiple of the tow vehicle""s depth. Alternatively, the amount of cable paid out is assumed to be the slant range to the tow vehicle. The inverted SLB method does not take into account the cable""s cantilever. Neither the lay-back the LBL, nor the SLB methods take in account the effects of local current on the cables"" shape and position. The accuracy of these methods will in general suffer as a function of time and therefore not support precise positioning of the towed vehicle.
The LBL method uses a series of acoustic transponders to localize the tow-vehicle by measuring the time delays between the tow-vehicle and the transponders. These transponders may be bottom mounted or localized on the surface, as shown in U.S. Pat. No. 5,119,341. While these systems are very accurate, their deployment is a time consuming and expensive operation. Furthermore the LBL method requires that the systems be redeployed to each operational area.
The SBL method uses time delay and measured arrival angle of an acoustic signal from the tow-vehicle to the towing vehicle to compute position. These systems are fairly accurate but their resolution is range dependent. Typically, in order to achieve sufficient accuracy, a SBL system is deployed over the tow-vehicle, requiring a second surface craft-an obvious economic disadvantage.
With the inverted SBL method, the directional acoustic receiver is mounted on the towed vehicle instead of the towing vehicle. As a consequence the range dependent accuracy of this system is a problem for very deep water operation. SBL and inverted SBL systems cannot be readily used in applications other than ship towed systems.
The LOST-1, localization method, generates the position of a towed underwater vehicle via the use of bathymetry data collected by the towing vessel and the depth range of the underwater vehicle from the towing vessel as shown in U.S. Pat. No. 6,256,264 incorporated herein by reference. LOST-1 uses a single dimensional model to generate the underwater vehicle""s position assuming the underwater vehicles position along track does not change. This feature limits the deployment of the underwater vehicle to an area directly behind the tow vessel in order to achieve sufficient accuracy.
A system for the accurate determination of the position of an underwater vehicle comprising a system observer subsystem having a state velocity update module, a terrain matching module, means for generating a prediction of the terrain matching module""s performance and a constrained extended Kalman filter subsystem. The constrained extended Kalman filter subsystem includes a steady state extended Kalman filter, a non-linear constraint module, and a state predictor. The system observer integrates bathymetry data corresponding to the area of the submersible vehicle, with the vessel""s measured ocean depth, the vessel""s predicted state, and the vessel""s measured velocity into a terrain based state estimate, and a final predicted state. The Kalman filter takes the terrain based state estimate, the final predicted state, the measured slant range and the location of the known point and computes the final estimate of the vessel""s position and a prediction of the vessel""s position at the next time step. A method for the accurate determination of the position of at least one underwater vehicle comprising the steps of (1) acoustically coupling at least one underwater vehicle to a sea borne position marker having a known position; (2) predicting the at least one underwater vehicle""s position, based on a past estimate of the underwater vehicle""s position, and an estimate of its velocity over the sea bottom; (3) estimating the underwater vehicle""s position utilizing measured ocean depth at the underwater vehicle""s position, bathymetry data and the underwater vehicle""s predicted position in a single point terrain match; (4) computing a estimate of the underwater vehicle""s position based on the prediction of the at least one underwater vehicle""s position based on vehicle dynamics and the estimated underwater vehicles position based on ocean depth and bathymetry data; and (5) computing a corrected estimate of the at least one submersible vehicle""s position that utilizes the estimate of the underwater vehicle""s position and a measured slant range from the at least one submersible vehicle to the sea borne position marker whose position is known.